Generally, there are two ways of testing the durability of a tire: (i) mounting the tire (i.e., a set of tires) on a vehicle and driving it on an actual roadway (this is called “road test”), and (ii) mounting the tire in a test machine which imposes stresses (e.g., radial loading) on the tire and running the tire on the test machine (this is called “laboratory test”). In both cases, measurements can be made while operating (driving, running) the tire, such as measuring tire temperature, stresses imposed upon a wheel (rim) to which the tire is mounted, etc. Additional observations can also be made after operating the tire (including during pauses in operating the tire), such as visual analysis of the condition and appearance of the tire. An example of a test machine 100 for is shown in FIG. 1.
A tire 102 being tested is mounted to a wheel (rim) 104 and is inflated to normal operating pressure (e.g., 32 psi; 220 kPa). The rim 104 rotates on an axle 106, about an axis of rotation. The tire is either free to rotate (passive, driven), or is rotated (active, driving) by a variable speed motor (not shown) driving the axle 106—hence rotating the tire 102. The tire 102 is typically a light passenger tire having an outside diameter of approximately 24″–30″ (609 mm–792 mm). However, it is within the terms of the present invention to use larger tires as required. A braking device (not shown), such as a simple disc brake (or dashpot), may be provided on the axle 106 to provide drag when the tire 102 is freewheeling (passive).
A road wheel 122 is provided. The road wheel 122 has a larger diameter than the tire—typically approximately 67″ (1.7 m). The road wheel 122 rotates on an axle 126, about an axis of rotation. The axle 126 is nominally parallel to the axle 106. The road wheel 122 is either free to rotate (passive), or is rotated (active) by a variable speed motor (not shown) driving the axle 126—hence rotating the road wheel. The road wheel 122 may have a smooth surface, or the surface may have a topology (e.g., irregularities such as bumps, holes, grooves, etc). In this example, the road wheel 122 has a generally smooth surface, simulating a well paved roadway. A braking device (not shown), such as a simple disc brake (or dashpot), may be provided on the axle 126 to provide drag when the road wheel 122 is freewheeling (passive).
The respective axles 106 and 126 are each supported by sets of bearing blocks (not shown) on either side of the tire 102 and wheel 122, respectively. If it is desired to move the axle, the respective sets of bearing blocks are movable by any conventional means, such as by a cylinder actuator or ball-and-screw connection. For example, the tire 102 may be moved in an out of engagement with the road wheel 122 by moving the tire axle 106 towards or away from the road wheel axle 126, as indicated by the arrows labeled a and a′. (Conversely, the tire axle 106 can remain stationary and the road wheel axle 126 can be moved towards or away from the tire axle 106.) More engagement equates to more radial load (radial with respect to the tire).
The bearing blocks (on either or both of the tire axle 106 or road wheel axle 126) can also be provided with strain gauges (not shown) for taking measurements of loads induced when the tire 102 is engaged against the road wheel 122. All of this can be under computer control (not shown) so that, for example, a bump in the road can be simulated by suddenly increasing, then decreasing the engagement of the tire and road wheel, and a dip or valley in the road can be simulated by decreasing, then increasing the engagement of the tire and road wheel, all according to a predetermined test regimen (protocol).
Additional degrees of freedom may be provided to simulate actual driving conditions in the laboratory. For example, the axle 106 can be pivoted (in the plane of the drawing sheet) about a point p at the center of the tire 102, as indicated by the arrows labeled b and b′. This simulates camber. By way of further example, the axle 106 can be mounted in the machine so that it can be rotated about an axis 108, as indicated by the arrow labeled c. This simulates steering forces.
Additional degrees of freedom may be provided, as desired, to simulate other tire driving conditions. For example, the axle 106 can be moved back and forth axially, as indicated by the arrows labeled d and d′ to simulate skidding conditions. Again, all of this can be under computer control (not shown) so that a wide range of actual driving conditions can be simulated, all according to a predetermined test regimen (protocol). And, as mentioned, sensors (not shown) can provide measurements during the test regimen.
In this example of a test machine 100, the tire axle 106 is shown as having all the degrees of freedom. It should be understood, as briefly mentioned above, that the road wheel axle 126 can be provided with comparable degrees of freedom, as may be desired.
A number of tires can be mounted on the test machine—for example, at least two tires on at least two rims on the same axle (106), and running on the same road wheel (122). (A suitably wide road wheel would be required.) Or at least two tires on at least two rims on at least two circumferentially displaced axles, and running on the same road wheel. Or combinations of the above.
FIG. 1A shows a computer (COMPUTER) for controlling the test machine 100 and running the laboratory test. The usual input/output peripherals (KEYBOARD, DISPLAY, ETC.) are provided. The computer controls the motor(s) (MOTOR(S)) driving the tire axle 106 and/or the road wheel axle 126, the braking devices (BRAKING DEVICE(S)), if any, associated with the tire axle 106 and/or the road wheel axle 126, the actuator(s) (ACTUATOR(S)) controlling the degrees of freedom for the tire axle 106 and/or the road wheel axle 126, and gathers data from any sensors, gauges, etc. (SENSORS, GAUGES, etc.) associated with the tire 102 and/or road wheel 122.
As is relevant to the present invention, the purpose of a laboratory test is to obtain information about the durability of the tire. This can be for “internal” purposes, such as testing new (prototype) tire designs, and can also be for obtaining acceptance (“certification”, if you will) of an existing (or a proposed) production tire design.
A road test currently in use and having acceptance in the industry is the “GM-ATE” test which is performed on public highways in Texas. (The ATE road test is specified by General Motors (GM).) A problem with road testing is that it is time consuming, logistically challenging, and can readily be profoundly adversely affected by road hazards (debris on the highway) which may be encountered.
What is needed is a laboratory test which can produce results which are comparable to those obtained by the GM-ATE road test.